Direct injection engines are those in which fuel is injected inside the cylinders of the engine, thereby enabling the amount of fuel burned and the timing of injection to be controlled precisely. A problem with such engines is that deposits tend to build up on the intake valves to unacceptable levels thereby interfering with the closing, motion and sealing of the valves. The efficiency of the engine is thus reduced and maximum power is limited. This is particularly evident in those engines utilising closed crankcase ventilation.
WO2004/094573 A1 describes a way of addressing the above-described problem of intake valve deposits by employing a lubricating oil composition comprising a base oil mixture, the base oil mixture comprising (i) a Group III oil, a Group IV oil, or a mixture thereof, in combination with (ii) a synthetic ester oil, the weight ratio of (i) to (ii) being from about 0.2:1 to about 6:1. It is to be noted that, in the examples of the aforesaid patent specification, each of the lubricating oil compositions contains a friction modifier.